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Space Exploration
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Discovery to Transport Last U.S., Boeing-built Starboard Truss
Segment to Space Station
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In the Space Station Processing Facility at NASA's Kennedy Space Center, Boeing workers stand ready as
the starboard integrated truss, known as S6, is rotated in order to remove and replace lower deck batteries.
The delivery of the International Space
Station’s (ISS) final, major U.S. and
Boeing-built truss segment, Starboard 6
(S6), by Space Shuttle Discovery during
the STS-119 mission will not only signal
the station’s readiness to house a sixmember crew for conducting increased
science activities on-orbit but will also be
another great example of American
ingenuity. With its two Solar Array Wings
(SAWs) for converting solar energy into
electrical power and a radiator for
rejecting heat away from electrical
components, the S6 is the final truss
element and completes the station’s 11segment integrated truss structure (ITS).
Also called a Photovoltaic Module (PVM)
because of its ability to generate, store
and distribute electrical power to the
station, the Starboard 6 segment will
ensure the outpost is powered to its
intended maximum potential.
A unique feature about the segment is
that it will carry two spare Battery
Charge/Discharge Units (BCDUs) used
for controlling the charge and discharge
of spare batteries on the outpost. The S6
segment was modified to carry the
additional payload of the BCDUs,
attached the segment’s Long Spacer
Truss structure, to increase the amount
of payload equipment that can be
transported to the space station during
shuttle missions.
The ISS solar arrays are the largest
deployable space assemblies ever built
and the most powerful electricityproducing arrays in orbit. Until deployed
following on-orbit installation, each SAW
remains folded in a special canister
called a Solar Array Assembly (SAA) that
is located at the end of S6 element. In
the canister, each wing is equipped with
an expandable mast. Two solar array
blanket boxes, containing a total of
32,800 solar cells, are
connected to the ends
of each canister and
are restrained to the
element frame for
launch. The addition of
S6 brings the station’s
total SAWs to eight.
Each wing is 115 by
38-feet wide and, when
all eight are fully
deployed, will
encompass an area of
32,528 square feet,
minus the masts.
item for its twin sister, the Port 6 Truss
Segment that was launched during
Space Shuttle Endeavour’s STS-97 on
Nov. 30, 2000. The starboard element
was delivered to Kennedy Space Center
in Florida on Dec. 17, 2002.
The space station is the greatest
construction project of humankind, and
as the prime contractor for the ISS,
Boeing designed the S6 and worked with
major subcontractors Lockheed Martin,
Honeywell, Space Systems/Loral, and
Hamilton Sundstrand to build it. Pratt and
Whitney Rocketdyne (which was a part
of Boeing at the time) provided most of
the electrical power system components.
Middeck Payload
In addition to S6, Space Shuttle
Discovery will carry in its middeck
several insulating sleeves to protect
some ungrounded connectors already
on-orbit, Crew Health Care System
(CHeCS) hardware items and Crew
provisioning items,
S6 Specifications
Environmental Control
Width: 16.3 feet; and Life Support
Dimensions:
195.48 inches
System (ECLSS)
hardware items for
Length: 45.4 feet; coolant sampling and
545.16 inches
water recovery, special
Extravehicular Activity
Height: 14.7 feet; (EVA) tools to support
176.54 inches
assembly operations,
and other resupply
31,060 lbs
On-Orbit Weight:
hardware.
Cost
The 310-foot integrated truss structure
that the S6 will be attached to forms the
backbone of the space station with
mountings for unpressurized logistics
carriers, radiators, solar arrays, and the
various elements. The 45-foot-long
skeletal S6 edifice is Discovery’s primary
payload and began as an operations test
Also, hardware for two
experiments will be
carried in the middeck:
1). The Middeck powered General
Laboratory Active Cryogenic and ISS
Experiment Refrigeration (GLACIER),
GLACIER ISS Utilization Payload, will be
swapped with a similar GLACIER being
returned from the ISS. GLACIER
GLACIER is a double locker cryogenic
freezer for transporting and preserving
science experiments that will remain on-
$297,918,471
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orbit at the end of the mission. The
freezer provides thermal control between
+4o Celsius and -160o Celsius and can
operate in both the space shuttle’s
Middeck and the EXPRESS Rack onorbit.
2). The Middeck powered Protein
Crystallization Diagnostic Facility –
Processing Unit (PCDF-PU) is being
flown up as an insert in an ISS locker.
During docked ops, the PCDF-PU insert
will be transferred to the ISS; for return,
the remaining locker shell will be used
for stowage of non-powered return items.
PCDF-PU is a multi-user facility for the
investigation of protein crystal growth
and other biological macromolecules
under microgravity.
configuration of the ISS. The S6 element
will then be handed to the Shuttle
Remote Manipulator System (SRMS or
shuttle arm) and maneuvered to another
location while the SSRMS changes base
points. The S6 will then be handed back
to the SSRMS and then maneuvered to
an overnight park position. The act of
removing the element from the payload
bay and maneuvering to an overnight
park position takes an entire day. The
following day the element will be
installed during a planned spacewalk.
Once the final truss segment is attached,
S6 will support power generation and
energy storage, utility routing, power
distribution and spare Orbital
Replacement Unit (ORU) storage.
Integrated Truss Segments and
Payload Structure
The integrated truss segments started
with Starboard zero (S0) as the center
assignment and were numbered in
ascending order outward to the port (P)
and starboard (S) sides. Starboard is the
right side and port is the left side of the
truss structure. Zenith (Z) and is up,
when the station is flying in its normal
direction, in its normal orientation. At
one time, there was an S2 and a P2
planned, but they were eliminated when
the station design was scaled back.
From S0, the truss segments are P1, P3,
P4, P5 and P6 and S1, S3, S4, S5 and
S6. P6 is attached to P5, and once onorbit S6 will be attached to S5. The S6
primary structure is made of a
hexagonal-shaped aluminum structure
and includes four bulkheads and six
longerons, which are beams that connect
the bulkheads.
Major Subsystems
Major subsystems of the S6 truss are the
starboard outboard Photovoltaic Module
(PVM), the Photovoltaic Radiator (PVR),
the Long Spacer Truss (LST) and the
Modified Rocketdyne Truss Attachment
System (MRTAS). The S6 PVM includes
all equipment outboard of Segment S5,
namely the two Photovoltaic Array
Assemblies (PVAAs) and the Integrated
Equipment Assembly (IEA). The PVR
provides thermal cooling for the IEA. The
MRTAS is used to provide a structural
interface to the S5 truss element. Each
PVAA consists of a SAW and Beta
Gimbal Assembly (BGA).
During the STS-119 mission, S6 will be
removed from the payload bay with the
Space Station Remote Manipulator
System (SSRMS or station arm)
because the shuttle’s arm is unable to
remove the element with the current
Major Elements
Photovoltaic Module (PVMs)
S6 will be the fourth and final of the four
PVMs that convert sunlight to electricity
on orbit. The primary functions of the
power module are to collect, convert,
store, and distribute electrical power to
loads within the segment and to other
station segments. Electrical power is the
most critical resource for the station
because it allows astronauts to live
comfortably, safely operate the station
and perform complex scientific
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experiments. Since the only readily
available source of energy for spacecraft
is sunlight, technologies were developed
also use the electricity to recharge
onboard batteries for continuous sources
of electricity while the station is in the
Earth’s shadow. Once complete, the
station power system, consisting of U.S.
and Russian hardware and four
photovoltaic modules, will use between
80 - 100 kilowatts of power or about as
much as 42 average houses (defined as
2,800-square-feet of floor space using 2
kilowatts each). Some of the electricity is
needed to operate space station
systems, but once that is figured in, the
addition of the S6 will nearly double the
amount of power available to perform
scientific experiments on the station -from 15 kilowatts to 30 kilowatts.
PVM components were assembled by
The Boeing Company in Tulsa, Okla.,
and Lockheed Martin in Sunnyvale,
Calif., before final assembly and testing
by Boeing at Kennedy Space Center,
Fla.
to efficiently convert solar energy to
electrical power.
The PVMs use large numbers of solar
cells assembled onto solar arrays to
produce high power levels. NASA and
Lockheed Martin developed a method of
mounting the solar arrays on a "blanket"
that can be folded like an accordion for
delivery to space and then deployed to
their full size once in orbit. The cells are
made from purified crystal ingots of
silicon that directly convert light to
electricity for immediate use through a
process called photovoltaics.
Gimbals are used to rotate the arrays so
that they face the sun to provide
maximum power to the space station.
After the conversion process, the PVMs
Solar Array Wings (SAW)
There are two SAWs on the S6 module,
each deployed in the opposite direction
of the other. Each SAW is made up of
two solar blankets mounted to a common
mast. Before deployment, each panel is
folded accordion style into a Solar Array
Blanket Box (SABB) measuring 20
inches high and 15 feet in length. Each
blanket is only about 20 inches thick
while in this stored position. The mast
consists of interlocking battens that are
stowed for launch inside a Mast Canister
Assembly (MCA).
When deployed by the astronauts, the
SAW unfolds like an erector set. Like a
human torso, it has two arms when
mounted on S6, and they are rotated
outwards by astronauts during a
spacewalk so they can be fully deployed.
Because these blankets were stored for
such a long time, extensive testing was
conducted to ensure they would unfold
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properly in orbit so the blankets would
not stick together.
power per PVM with a total of four PVMs
on the station.
When fully deployed, the SAW extends
The solar arrays produce more power
115 feet and spans 38 feet across and
extends to each side of the Integrated
Equipment Assembly. Since the second
SAW is deployed in the opposite
direction, the total wing span is more
than 240 feet.
than can be made available to the
station’s systems and experiments.
Because all or part of the solar arrays
are eclipsed by the Earth or station
structure at times, batteries are used to
store electicity for use during those
periods. About 60 percent of the
electricity generated is used to recharge
the batteries. During long eclipse
periods, power availability is limited to
about 10.5 kilowatts from each SAW, or
30 kilowatts per PVM. During shorter
eclipse periods more power is available
to station systems and experiments.
Circuit breakers also regulate the flow of
electricity to prevent overheating of the
Utility Transfer Assembly (UTA) that
allows power to flow through the rotating
SARJ.
Each SAW weighs more than 2,400
pounds and uses 32,800 solar array
cells, each measuring 8 centimetes
square with 4,100 diodes. The individual
cells were made by Boeing’s Spectrolab
and Aviation Systems Engineering Co.
There are 400 solar array cells to a string
and there are 82 strings per wing. Each
SAW is capable of generating 32.8
kilowatts, or about 10.5 to 15 kilowatts of
usable power. There are two SAWs on
the S6 module capable of delivering a
combined 21 to 30 kilowatts of usable
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Space Station Power
Current
(3 PVMs)
198 kilowatts
66 kilowatts
264 kilowatts
Usable Power
Capability*
63-90 kilowatts
21-30 kilowatts
84-120 kilowatts
Power for
Science**
15 kilowatts
15 kilowatts
30 kilowatts
Power
Generation
Capability
Starboard 6
Total (Post STS-119)
* The amount of usable power varies depending on the time of year and the orientation of the
station relative to the Earth and sun.
** A greater fraction of power from the first three photovoltaic modules (PVMs) currently installed
is needed to support day-to-day station systems operation.
Solar Alpha Rotary Joint
(SARJ)
When the S6 truss becomes
attached to the S5 short
spacer, S6 will be positioned
by the starboard SARJ, which,
when fully operational, will
continuously rotate to keep the
solar array wings on S4 and
S6 oriented toward the sun as
the station orbits the Earth.
Located between S3 and S4,
the starboard SARJ is a 10.5foot diameter rotary joint
designed to track the sun in
the alpha axis that turns the
entire S4/S5/S6 module
assembly. The starboard
SARJ race ring has been
damaged from inadequate
lubrication from the gold rollers
on the Trundle Bearing
Assemblies that attach to the ring. The
race ring has a triangular cross-section
that 12 TBA bearings roll on. As a result
of cleaning and lubrication, the starboard
SARJ can perform automated
continuous tracking mode when needed.
Extensive simulated wear testing of the
race ring with lubricant is now being
performed to determine further actions, if
required. The SARJ weighs
approximately 2,500 pounds. The SARJ
can spin 360 degrees using bearing
assemblies and a servo control system
to turn. All of the power flows through the
Utility Transfer Assembly (UTA) in the
SARJ. Roll ring assemblies allow
transmission of data and power across
the rotating interface so it never has to
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unwind. Under contract to Boeing, the
SARJ was designed, built and tested by
Lockheed Martin in Sunnyvale, Calif.
Beta Gimbal Assembly (BGA)
The solar array wings also are oriented
by the BGA, which can change the pitch
of the wings by spinning the solar array.
The BGA measures 3 x 3 x 3 feet and
provides a structural link to the
Integrated Equipment Assembly (IEA.)
The BGA’s most visual functions are to
deploy and retract the SAW and rotate it
about its longitudinal axis. The BGA
consists of three major components: the
Bearing, Motor and Roll Ring Module
(BMRRM), the Electronics Control Unit
(ECU) and the Beta Gimbal Transition
Structure, mounted on the BGA Platform.
The BGA was designed by Boeing
Rocketdyne in Canoga Park, Calif.,
which has since been acquired by Pratt
and Whitney. The Sequential Shunt Unit
(SSU) that serves to manage and
distribute the power generated from the
arrays and is also mounted on each BGA
platform. The SSU was designed and
manufactured by Space Systems/Loral.
Both the SARJ and BGA are pointing
mechanisms and mechanical devices
used to point the arrays toward the sun.
They can follow an angle target and
rotate to that target in the direction
toward the sun On-orbit controllers
continuously update those targets so
they keep moving continuously as the
station orbits the Earth every 90 minutes,
maintaining contact with the sun at the
same orbital rate. The SARJ mechanism
moves much more than the BGA, which
moves about four or five degrees per
day, whereas the SARJ rotates 360
degrees every orbit or about 4 degrees
per minute.
S6 Integrated Equipment Assembly
(IEA)
The IEA has many components: 12
battery subassembly Orbital
Replacement Units (ORUs), Battery
Charge/Discharge Units (BCDU) ORUs,
two Direct Current Switching Units
(DCSUs), two Direct Current to Direct
Current Converter Units (DDCUs), and
two Photovoltaic Controller Units
(PVCUs). The IEA integrates the
Thermal Control Subsystem that consists
of one Photovoltaic Radiator (PVR) ORU
and two Pump Flow Control
Subassembly (PFCS) ORUs, which are
used to transfer and dissipate heat
generated by the IEA ORU boxes.
In addition, the IEA provides
accommodation for ammonia servicing of
the outboard PV modules as well as
pass through of power and data to and
from the outboard truss elements. The
IEA measures 16 x 16 x 16 feet, weighs
nearly 17,000 pounds and is designed to
condition and store the electrical power
collected by the photovoltaic arrays for
use on board the station. The IEA
integrates the energy storage
subsystem, the electrical distribution
equipment, the thermal control system
and structural framework. The IEA
consists of three major elements:
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The Boeing-built Starboard 6 truss segment and
its folded solar arrays are loaded into the payload
canister for its trip to the launch pad. Boeing
workers played a key role in keeping the element
ready for launch, to include swapping out the
batteries with a fresh set.
1. The power system electronics
consisting of the Direct Current
Switching Unit (DCSU) used for primary
power distribution; the Direct Current to
Direct Current Converter Unit (DDCU)
used to produce regulated secondary
power; the Battery Charge/Discharge
Unit (BCDU) used to control the charging
and discharging of the storage batteries;
and the batteries used to store power.
2. The Photovoltaic Thermal Control
System (PVTCS) consisting of: the
coldplate subassemblies used to transfer
heat from electronic boxes to the
coolant; the Pump Flow Control
Subassembly (PFCS) used to pump and
control the flow of ammonia coolant; and
the Photovoltaic Radiator (PVR) used to
dissipate the heat into deep space.
Ammonia, unlike other chemical
coolants, has significantly greater heat
transfer properties.
3. The computers used to control the S6
module ORUs consisting of two
Photovoltaic Controller Unit (PVCU)
Multiplexer/Demultiplexers (MDMs).
The IEA power system is divided into two
independent and nearly identical
channels. Each channel is capable of
control (fine regulation), storage and
distribution of power to the station. The
two SAWs are attached to the outboard
end of the IEA.
Direct Current Switching Unit (DCSU)
Power received from each SAW is fed
directly into the appropriate DCSU, a
high-power, multi-path remotely
controlled unit used for primary and
secondary power distribution, protection
and fault isolation within the IEA. During
periods of insolation ("insolation" means
during periods of sunlight), the DCSU
routes primary power directly to the
station from its SAW and also routes
power to the power storage system for
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battery charging. During periods of
eclipse, the DCSU routes power from the
power storage system to the station. The
DCSU measures 25” x 40” x 14” and
weighs 218 lbs.
Direct Current to Direct Current
Converter Unit (DDCU)
Primary power from the DCSU also is
distributed to the DDCU, a power
processing system that conditions the
coarsely regulated power from the SAW
and BCDUs to 124.5 +/- 1.5 VDC. It has
a maximum power output of 6.25 kW.
This power is used for all S6 operations
employing secondary power. By
transmitting power at higher voltages
and stepping it down to lower voltages
where the power is to be used, much like
municipal power systems, the station can
use smaller wires to transmit this
electrical power and thus reduce launch
loads. The converters also isolate the
secondary system from the primary
system and maintain uniform power
quality throughout the station. The
DDCU measures 27.25” by 23” by 12”
and weighs 129 pounds.
Primary power from the DCSU also is
distributed to the three power storage
systems within each channel of the IEA.
The power storage system consists of a
Battery Charge/Discharge Unit (BCDU)
and two battery subassembly ORUs. The
BCDU serves a dual function of charging
the batteries during solar collection
periods and providing conditioned
battery power to the primary power
busses (via the DCSU) during eclipse
periods. The BCDU has a battery
charging capability of 8.4 kW and a
discharge capability of 6.6 kW. The
BCDU also includes provisions for
battery status monitoring and protection
from power circuit faults. Commanding of
the BCDU is from the PVCU. The BCDU
measures 28” by 40” by 12” and weighs
235 pounds.
Each battery subassembly ORU consists
of 38 lightweight nickel hydrogen cells
and associated electrical and mechanical
equipment. Two battery subassembly
ORUs connected in series are capable of
storing 8 kWh (kilowatt-hours) of
electrical power. This power is fed to the
station via the BCDU and DCSU
respectively. The batteries have a design
life of 6.5 years and can exceed 38,000
charge/discharge cycles at 35 percent
depth of discharge. Each battery
measures 41” by 37” by 19” and weighs
372 pounds.
Photovoltaic Thermal Control System
(PVTCS)
To maintain the IEA electronics and
batteries at safe operating temperatures
in the harsh space environment, a
PVTCS is used. The PVTCS consist of
ammonia coolant, 11 coldplates, two
Pump Flow Control Subassemblies
(PFCS) and one Photovoltaic Radiator
(PVR).
The coldplate subassemblies are an
integral part of the IEA structural
framework. Heat is transferred from the
IEA orbital replacement unit (ORU)
electronic boxes to the coldplates via fine
interweaving fins located on both the
coldplate and the electronic boxes. The
fins add lateral structural stiffness to the
coldplates in addition to increasing the
available heat transfer area.
Pump Flow Control Subassemblies
(PFCS)
The PFCS is the heart of the thermal
system, consisting of all the pumping
capacity, valves and controls required to
pump the heat transfer fluid to the heat
exchangers and radiator, and regulate
the temperature of the thermal control
system ammonia coolant. The PVTCS is
designed to dissipate and average of
6,000 Watts of heat and communicates
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with the PVCUs. Each PFCS consumes
275 Watts during normal operations and
measures approximately 40 x 29 x 19
inches, weighing 235 pounds.
Photovoltaic Radiator (PVR)
The PVR – the radiator – is deployable
on orbit and comprised of two separate
flow paths through seven panels. Each
flow path is independent and is
connected to one of the two PFCSs on
the IEA. In total, the PVR can reject up to
14 kW of heat into deep space. The PVR
weighs 1,633 pounds and when
deployed measures 44 x 12 x 7 feet.
Contact:
Susan Wells
Space Exploration (Florida)
(321) 264-8580
susan.h.wells@boeing.com
Adam Morgan
Space Exploration (Houston)
(281) 226-4030
adam.k.morgan@boeing.com
Last Updated: January 2009
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